Increasing concern with atmospheric pollution has led to the establishment of standards for known polluting materials present in stack gases, including nitrogen oxides. Nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are especially important because they react in the presence of sunlight to form a number of complex compounds which are significant contributors to air pollution and the formation of smog. Nitric oxide (NO) is the principal nitrogen oxide formed during the high temperature reaction between air and hydrocarbon fuels. However, at lower temperatures and in the presence of excess air, nitric oxide (NO) may be converted to nitrogen dioxide (NO.sub.2). The ratio of the two oxides varies depending upon the number of variables, e.g., sunlight, oxygen, or other oxidizing or reducing agents, both oxides usually being lumped together and termed NO.sub.x.
The present invention relates to a method and apparatus for reducing NO.sub.x resulting from combustion of nitrogen containing fuels. NO.sub.x is produced no matter what type of fuel is used. This is true even though a fuel is burned which does not inherently contain nitrogen as one of its components, e.g. natural gas which is essentially pure methane. Combustion products from a nitrogen-free fuel still contain NO.sub.x, which has been derived from the molecular nitrogen introduced as air into the combustion process. The NO.sub.x resulting from the nitrogen in the air may be termed "thermal NO.sub.x ". Many heavier oils and coal which are commonly used for industrial purposes contain nitrogen compounds in varying amounts. These nitrogen compounds also produce NO.sub.x as part of the combustion process in addition to the NO.sub.x from atmospheric nitrogen. Since nitrogen-containing fuels produce more NO.sub.x than nitrogen-free fuels, it is the reduction of nitrogen NO.sub.x resulting from "fuel NO.sub.x " which is the principal object of the present invention.
There are many ways which may be employed to reduce NO.sub.x. These may be grouped under at least three major categories: (1) Control of the fuel nitrogen; (2) treatment of stack gases; and (3 control of the combustion process by adjusting the key variables, i.e., oxygen, temperature, mixing and residence time. The present invention deals with a novel burner for control of the combustion process within the third category. A general discussion of the control of nitrogen oxide emissions in combustion may be found in a paper presented by William Bartok et al at the International Congress of Chemical Engineering at the Service of Mankind, European Federation of Chemical Engineering, Paris, France, Sept. 2-9, 1972.
Since nitrogen oxides are produced by the reaction of nitrogen with oxygen, one approach which can be taken is to limit the availability of oxygen for such a reaction. This is complicated by the fact that oxygen is required for the combustion of the fuel and generally must be used in excess in order to assure complete combustion. Burning with a limited air supply to create reducing conditions and thus to minimize the production of nitrogen oxides has been utilized in the prior art, particularly to destroy relatively large quantities of nitrogen oxides which had been formed from chemical processing. Typical of such prior art processes are the following:
British Pat. No. 1,274,637 -- Robert D. Reed et al.
U.S. Pat. No. 2,673,141 -- Barman
U.S. Pat. No. 3,505,027 -- Breitbach et al.
U.S. Pat. No. 3,661,507 -- Breitbach et al.
Formation of nitrogen oxides is favored by high temperatures. Thus, much of the prior art effort has been directed to reducing combustion temperatures in order to reduce NO.sub.x formation. This is, of course, directionally undesirable inasmuch as industrial furnaces operate with greater efficiency when high combustion temperatures are used. However, direct cooling techniques, e.g. flue gas recirculation and water injection, are effective methods of limiting combustion temperatures and thereby the NO.sub.x production. A staged combustion technique incorporating indirect cooling of the flue gases has been used for stationary power boilers as disclosed in U.S. Pat. No. 3,048,131 to Hardgrove. It should be noted that in utility boilers excess air is closely controlled in order to maximize the efficiency of the heat transfer process. These high temperatures result in excessive NO.sub.x production but, combustion at below stoichiometric conditions, which inherently occurs at lower temperatures and under semireducing conditions, will limit the NO.sub.x production. If this first combustion step is followed by cooling of the flue gases, combustion may be completed by addition of air while keeping temperature low and limiting NO.sub.x production. This has been accomplished in utility boilers by firing burners at the bottom of the boiler with sub-stoichiometric air to fuel ratios. By the time the flue gases which are produced have reached the upper portion of the boiler, the temperature has been reduced by cooling against the steam generating tubes and air may be introduced to complete the combustion process. A variation of the staged combustion process has been employed by Livingston (U.S. Pat. No. 3,356,075) and by Bienstock et al (U.S. Pat. No. 3,382,822) in the firing of boilers with coal.
The foregoing prior art was directed mainly to reducing NO.sub.x production in utility boilers which by their construction lend themselves to the application of staged combustion. Industrial furnaces used in the petroleum industry are not so simply modified. The physical size and shape of such furnaces is determined by the flames produced by the burners used. Modification of existing furnaces to reduce their NO.sub.x production should preferably be done by replacing burners. Newly designed furnaces should preferably incur a minimum of added expense and a minimum loss of efficiency, while operating with substantially lower NO.sub.x production. An improved furnace must also operate to reduce NO.sub.x produced by the high nitrogen content fuels which are often used. The present invention disclosed herein is an improved method and apparatus whereby a reduction of NO.sub.x production from industrial furnaces may be obtained.